1. Technical Field
The present invention relates to monitoring and control of subsurface installations located in one or more reservoirs of fluids such as hydrocarbons, and more particularly to methods and installations for providing wireless transmission of power and communication signals to, and receiving communication signals from, those subsurface installations.
2. Related Background Art
Reservoir monitoring includes the process of acquiring reservoir data for purposes of reservoir management. Permanent monitoring techniques are frequently used for long-term reservoir management. In permanent monitoring, sensors are often permanently implanted in direct contact with the reservoir to be managed. Permanent installations have the benefit of allowing continuous monitoring of the reservoir without interrupting production from the reservoir and providing data when well re-entry is difficult, e.g. subsea completions.
Permanent downhole sensors are used in the oil industry for several applications. For example, in one application, sensors are permanently situated inside the casing to measure phenomenon inside the well such as fluid flow rates or pressure.
Another application is in combination with so-called smart or instrumented wells with downhole flow control. An exemplary smart or instrumented well system combines downhole pressure gauges, flow rate sensors and flow controlling devices placed within the casing to measure and record pressure and flow rate inside the well and adjust fluid flow rate to optimize well performance and reservoir behavior.
Other applications call for using sensors permanently situated in the cement annulus surrounding the well casing. In these applications, formation pressure is measured using cemented pressure gauges; distribution of water saturation away from the well using resistivity sensors in the cement annulus; and seismic or acoustic earth properties using cemented geophones. Appropriate instrumentation allows other parameters to be measured.
These systems utilize cables to provide power and/or signal connection between the downhole devices and the surface. The use of a cable extending from the surface to provide a direct to connection to the downhole devices presents a number of well known advantages.
There are however, a number of disadvantages associated with the use of a cable in the cement annulus connecting the downhole devices to the surface including: a cable outside the casing complicates casing installation; reliability problems are associated with connectors currently in use; there is a risk of the cable breaking; the cable needs to be regularly anchored to the casing with cable protectors; the presence of a cable in the cement annulus may increase the risk of an inadequate hydraulic seal between zones that must be isolated; added expense of modifications to the wellhead to accommodate the feed-through of large diameter multi-conductor cables; the cables can be damaged if they pass through a zone that is perforated and it is difficult to pass the cable across the connection of two casings of different diameters.
In efforts to alleviate these and other disadvantages of downhole cable use, so-called xe2x80x9cwireless systemsxe2x80x9d have been developed.
Bottom electromagnetic telemetry allows for electrical signals to be injected into conductive casings to create an electrical dipole source at the bottom of the well in order to telemeter measurement data from the subsurface to the surface. A related idea uses currents in a casing segment downhole to establish a magnetic field in the earth, the latter used to steer another well being drilled.
Bottom switching as telemetry via casing and tubing or wireline utilizes various arrangements of an electrical switch downhole between casing and tubing, between casing and a wireline tool, or between two electrically isolated segments of casing to send downhole measurement data to a surface detection and recording system.
Tubing-Casing transmission (xe2x80x9cTUCASxe2x80x9d), a wireless two-way communication system, developed and patented by Schlumberger (U.S. Pat. No. 4,839,644 which is incorporated herein by reference), in which an insulated system of tubing and casing serve as a coaxial line as illustrated in FIG. 1. Both power and two-way signal (communication) transmission are possible in the TUCAS system. Because the system uses an inductive coupling technique to inject or retrieve power and signal from the system, only on the order of several tens of watts of power can be sent to the downhole sensor devices, which is adequate for commercial pressure gauge sensors. Additionally, electrical insulation between the tubing and casing must be maintained.
Likewise, shortcomings are evident in known systems where a toroid is used for current injection in casing or a drill string which is in contact with a surrounding cement annulus or earth formation. In addition to the limitations on the level of power which can be inductively coupled, the current loop will be local as the current return will seek the shortest electrical path through the formation to return to casing, as illustrated in FIG. 2.
Another system using casing conductivity injects current for locally heating the formation to help move viscous hydrocarbon fluids. This system, as illustrated in FIG. 3, concentrates a large current into a minimal area resulting in localized high current density in the resistive earth, thereby generating heat. High current density is seen in heated zone H while very low current density is seen at surface return electrode R.
A simple surface return is utilized as there is no concern with overall system efficiency as far as electrical circulation is concerned. This type of system does not use the casing in conjunction with downhole electronics, i.e. for communication with or direct power transfer to downhole electronics, but rather focuses on the generation of heat in the formation via concentration of a large current flux at the end of the casing in zone H. Insulation is employed for current concentration in zone H by preventing injected current from flowing out of the casing to the surrounding formation except where desiredxe2x80x94i.e., at the bottom of the well where the casing is exposed in zone H.
Several practical disadvantages are evident in such a system as that of FIG. 3. One primary, and potentially dangerous disadvantage is that the wellhead is necessarily maintained at a very high potential in order to achieve the desired current density at well bottom to generate sufficient formation heating for their desired purposes. This can pose significant danger to the crew at the well site.
Limitations of the prior art are overcome by the method and apparatus of the present invention of power and signal transmission using insulated casing for permanent downhole installations as described hereinbelow.
The present invention is directed to various methods and apparatus for transmitting at least one electrical signal to or from at least one downhole device in a well. The method comprises providing an electrically conductive conduit in the well, electrically insulating a section of the conduit by encapsulating a section of the conduit with an insulative layer and insulating the encapsulated section of conduit from an adjoining section of the conduit by using a conduit gap, introducing the electrical signal within the insulated section of conduit, providing a return path for the electrical signal, and connecting the downhole device to the insulated section.
In alternative embodiments, the method includes introducing the electrical signal is performed via inductive coupling and/or direct coupling. The electrical signal includes power or communication signals. The electrical signals can be introduced by one of the downhole devices or by a surface device, directly or inductively coupled to the insulated section of conduit. The method may also include use of a second conduit gap to form a completely electrically insulated conduit section. In the various embodiments, single or multiple devices may be coupled to the insulated section of conduit. The return path for the electrical signal may be provided through the earth formation surrounding the well, through the cement annulus or through an outer conductive layer of the conductive conduit. An apparatus is also disclosed for transmitting at least one electrical signal to or from at least one downhole device in a well. In various embodiments, the apparatus comprises an electrically conductive conduit installed in the well, insulation means for electrically insulating a section of the conduit, the insulation means comprising an insulative encapsulation layer around the section of the conduit and a conduit gap insulating the insulated section of the conduit from an adjoining section of the conduit, means for introducing the electrical signal within the insulated section of the conduit, means for providing a return path for the electrical signal, and means for electrically connecting the downhole device to the insulated section of the conduit. In alternative embodiments, the apparatus comprises inductive coupling and/or direct coupling for introducing the electrical power or communication signals. The electrical signals can be introduced by one of the downhole devices or by a surface device, directly or inductively coupled to the insulated section of conduit. The apparatus may also comprise a second conduit gap to form a completely electrically insulated conduit section. In the various embodiments, single or multiple devices may be coupled to the insulated section of conduit. The return path for the electrical signal may be provided through the earth formation surrounding the well, through the cement annulus or through an outer conductive layer of the conductive conduit.
The foregoing and other features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawings.